The invention refers to a pump for pumping a fluid comprising liquefied gas. A pump of this kind is provided with an inlet and an outlet for the fluid and with a housing comprising an inner space, in which a piston is displaceably guided. The pump is also provided with fluid feeding means connecting said inlet with a pump chamber of said inner space provided between an end member of said housing and said piston and comprising a first back-pressure valve, furthermore with fluid abducting means connecting said pump chamber with said outlet and comprising a second back-pressure valve. A pump of this kind, also known as cryogenic pump, may be used for example for pumping a fluid comprising one or more liquefied gases, such as liquid nitrogen, liquid oxygen, liquid argon, or liquid air, and perhaps a small quantity of fluid in the gaseous state. However, the fluid to be pumped should be by preference in the liquid state, at least in the main. The pump should be capable of raising the pressure of the cryogenic fluid lying initially at or slightly above the pressure of the surrounding air, for example, and typically between 0.1 and 0.5 MPa, to a value lying for example between 20 and 50 MPa, or higher.
Single stage cryogenic pumps known on the market comprise an inlet and an outlet for the fluid to be pumped, a housing, a piston guided displaceably within said housing, and a drive device for displacing said piston. An end or bottom member of the housing, disposed on the side of the piston facing away from the drive device, comprises a first back-pressure valve, by way of which fluid supplied to the inlet may be sucked into the pump chamber provided between said end member and said piston. A jacket portion of the housing enclosing the pump chamber is provided at a peripheral location with a passage connected with the outlet of the pump by way of a second back-pressure valve. If during operation the piston moves away from said end member and pumps fluid into said pump chamber, the situation may arise, particularly during the beginning phase of a suction stroke, that--because of the under-pressure generated--a relatively large portion of the liquefied gas will become reconverted to the gaseous state, resulting in disturbed operation of the pump, a considerable disadvantage.
This disadvantage is known to be avoided by interposing a supercharger and making the pump two-staged. A pump of this type is known for example from U.S. Pat. No. 4,639,197, to comprise a housing subdivided by means of a dividing wall into two cylindrical inner spaces, each of which being arranged to accommodate a piston displaceably guided therein. The two pistons are fixedly mounted on a common piston rod passing through a hole of the dividing wall provided with a sealing element, and are connected with a drive device by way of said common piston rod. The piston located further away from the drive device serves, together with a sleeve of the housing enclosing the piston and open at its end facing away from the drive device, as a supercharger for pumping the liquefied gas, through the passages provided in the dividing wall, into the pump chamber associated with the high-pressure compressor. In order to prevent the portion of the piston rod connecting the two pistons of the pump, known from the pump of U.S. Pat. No. 4,639,197, from occupying too much space within the chambers provided between the separating wall and the two pistons, said portion of the piston rod should preferably be considerably thinner than the pistons. However, if it is dimensioned thin, the piston rod will be in considerable danger to break during operation. The dividing wall, the two pistons, and the piston rod portion connecting the two pistons, must be cooled down during operation, from the normal room temperature to temperatures lying--depending upon the fluids to be pumped--around -200.degree. C. Also, a large pressure difference develops during pumping between the supercharger and the pump chamber of the high-pressure compressor. Therefore, the sealing element required for sealing the passage of the piston rod through the dividing wall will be costly to make and susceptible to trouble during operation, especially if the portion of the piston rod that penetrates through the dividing wall is dimensioned relatively thin. Also, even if the piston rod is dimensioned thin, the space requirement of the pump--assuming a prescribed pumping output--will nevertheless increase; in fact, a two-stage pump, per se, requires more space than a single-stage pump of the same pumping output. The features requiring that the essential part of the dividing wall and the jacket of the housing enclosing the piston of the high-pressure compressor be located on the exterior of a vessel containing liquefied gas supplied through the inlet, result in the additional disadvantage of this pump, namely that the high-pressure compressor is not insulated with respect to its surroundings, and the heat generated by friction by the piston of the high-pressure compressor cannot be carried off adequately.